Force-sensing instruments are disclosed in U.S. Pat. Nos. 4,932,258 (issued to Norling on June 12, 1990, and not admitted to be prior art); U.S. Pat. No. 4,441,366 (issued to Hanson on Apr. 10, 1984); U.S. Pat. No. 4,399,700 (issued to Hanson on Aug. 23, 1983; and U.S. Pat. No. 4,394,405 (issued to Atherton on July 19, 1983). These patents are owned by Sundstrand Data Control, Inc. The precision electromagnetic wire coils of such instruments must be non-magnetic (i.e. not magnetic or paramagnetic) so they will not be influenced by ambient magnetic fields, such as the internal, permanent magnetic field of an accelerometer, the Earth's magnetic field, or magnetic fields surrounding nearby electrical equipment. The wire used in such coils is extremely fine, for example 44 to 46 gauge AWG (55.9 to 44.4 micron nominal diameter) magnet wire. The wire used to make such coils must have an extremely precise and constant weight, so the coils will have minimal variations in weight and electrical and mechanical characteristics over time, or from coil to coil.
Corrosion of the copper wire of such a coil by the oxygen in air; by wire-drawing and other lubricants; by outgassing of potting compounds, adhesives, and impurities in other parts; and by other environmental factors will significantly change its weight and conductivity during the service life of a device containing it, and thus affect the accuracy of the device. Such corrosion is made worse if the copper wire is exposed to heat and captive moisture, particularly if it is heated and cooled repeatedly. One example of cyclic heating is incorporation of such coils in well measurement tools used in downhole oil well drilling equipment. Downhole temperatures exceeding 150.degree. C. alternating constantly with ambient surface temperature are common in this environment.
When the copper wire of such a coil is heated, it tends to throw off impurities. When it cools, the copper wire takes up any impurities which may be present. Such impurities may accelerate corrosion, particularly in the welded joint between the wire and its terminals, and thus cause the joint to fail. Such corrosion must be minimized.
Conventional corrosion-inhibiting insulation, such as varnish, water glass (sodium silicate solution), or another non-metallic material is not suitable in this environment because the insulation must be stripped from the ends of the wire which are welded to terminal pads. Thus, the welded copper wire is exposed to air at the joint. Even if the ends of the wire are stripped of insulation and then plated with gold or another noble metal, the plating will not adhere to the insulation from which the stripped end extends. Thus, contaminants and corrosion can penetrate to the copper conductor between the point where the insulation ends and the point where the plating begins.
Another problem in the art is the strength of the weld between the end of a copper coil and a gold terminal pad, for example, the gold terminal pads or plating to which the ends of the coils referred to herein are welded. Moderate loads on the coil in the field may break this weld and cause the equipment to fail.
Yet another problem in the art is the need to protect a coil and its joints with terminal pads from corrosion without appreciably changing the weight or mechanical, electrical, or magnetic characteristics of the coil, so the attached mechanical elements do not need to be redesigned for a new coil. For example, wires made of solid noble metal, such as gold, would weigh more and be less conductive for a given cross-section than copper wires. Wires made of nickel would be paramagnetic, which is undesirable.
The present assignee has previously formed a welded joint directly between a gold terminal pad and the usual insulated copper magnet wire of a precision electromagnetic coil. In one technique (which is not admitted to be prior art), the end of the wire to be joined was stripped, and a stripped portion of its cylindrical surface at or near its end was laid on the terminal pad and welded to it. In the finished joint, the stripped end of the wire often protruded from the joint. This joint was made of dissimilar metals, one of which was non-noble, so the joint was subject to galvanic corrosion. The copper wire in and adjacent to the joint was exposed to outgassing from adjacent components, captive air and moisture, and the like, and thus was also prone to chemical corrosion.
The art also teaches that electrical conductors, particularly the conductor patterns on printed and integrated circuits, can be corroded by their environments, even by ordinary air, so they should be plated with a less easily corroded (i.e. more noble) metal. Nickel, gold, and other metals are known to be useful as such coatings. The prior art also teaches that noble metals per se, and particularly gold, are desirable electrical conductors in microcircuit applications in which the total quantity of gold used is small, so its weight and expense are not significant.
U.S. Pat. No. 4,238,300 (Yoshida), column 1, teaches that copper headers and housings for semiconductor diodes, which would be contaminated by copper ions, can be electroplated with gold. At column 4, Yoshida teaches "hard" gold plating bath formulations (containing cobalt or nickel hardening additives) and "soft" gold plating bath formulations which can be used to plate gold.
U.S. Pat. No. 4,001,093, issued to Koontz et al. on Jan. 4, 1977, teaches that precious metals like gold can be plated on localized parts of electrical components, such as connectors and switches. The reference states that surface contact to gold usually has low electrical resistance, and that the small-dimensioned conductor lines of copper printed circuits initially may have acceptable conductivity, but rapidly degrade with time. The reference also states that, by adding small amounts of arsenic, copper, or nickel to gold, the gold can be made quite hard and resistant to abrasion.
U.S. Pat. No. 4,002,778, issued to Bellis et al. on Jan. 11, 1977, teaches that nickel or cobalt can be plated onto the conductor pattern of a printed circuit.